One of the biggest problems caused from vehicles using fossil fuel, such as gasoline and diesel oil, is creation of air pollution. A technology of using a secondary battery, which can be charged and discharged, as a power source for vehicles has attracted considerable attention as one method of solving the above-mentioned problem. As a result, electric vehicles (EV), which are operated using only a battery, and hybrid electric vehicles (HEV), which jointly use a battery and a conventional engine, have been developed. Some of the electric vehicles and the hybrid electric vehicles are now being commercially used. A nickel-metal hydride (Ni-MH) secondary battery has been mainly used as the power source for the electric vehicles (EV) and the hybrid electric vehicles (HEV). In recent years, however, the use of a lithium-ion secondary battery has been attempted.
High output and large capacity are needed for such a secondary battery to be used as the power source for the electric vehicles (EV) and the hybrid electric vehicles (HEV). For this reason, a plurality of small-sized secondary batteries (unit cells) are connected in series or in parallel with each other so as to construct a battery pack.
FIG. 1 is a typical view schematically illustrating a conventional battery pack including unit modules having the same output and capacity characteristics.
Referring to FIG. 1, a conventional battery pack 100 includes a unit module 120 comprising a plurality of battery cells 110 having the same output and capacity, a control unit 130 for controlling the operation of the unit module 120, and an external input and output terminal 140 for connecting the control unit 130 to a device (not shown).
As shown in FIG. 1, the conventional battery pack 100 includes only one kind of unit module 120 (a mechanical and electrical connection assembly having same kind of unit cells). However, the initial operating condition of a device is generally different from the intermediate operating condition of the device. As a result, the conventional battery pack 100 has a limit in that the conventional battery pack 100 cannot efficiently respond to those operating conditions.
At the present time, most battery packs include one kind of battery cells, i.e., large-capacity battery cells or high-output battery cells, as unit cells. However, these battery packs are not suitable for a system requiring high output for the starting torque and large capacity after the starting, like a motor. In other words, battery packs including only one kind of unit module cannot be used in a system requiring complex performance.
Consequently, a device (or a system) requiring complex operating modes needs a plurality of battery packs. In the case of using the plurality of battery packs, however, the spatial and time efficiencies and the safety are lowered. Furthermore, a high-level technology and a large amount of costs are needed to develop a battery cell itself satisfying all the characteristics.
In consideration of the above-described matter, it is urgently required to develop a novel battery pack that can be applied to a system requiring complex characteristics and has improved spatial efficiency.
In this connection, there has been known a technology using a fuel cell as a hybrid power supply unit. For example, Japanese Unexamined Patent Publication No. H6-124720 discloses a battery system including two or more storage batteries and a fuel cell. According to this battery system, power is supplied from one storage battery to the outside while power is supplied from the fuel cell to another storage battery such that the another storage battery can be charged, whereby stable power supply is accomplished. Although the battery system is a battery pack including a plurality of batteries, the principal component of the battery system is the fuel cell. As a result, the disclosed battery system cannot be efficiently operated depending upon the operating conditions of a device in which the battery system is mounted.
In addition, Japanese Unexamined Patent Publication No. 2004-111242 discloses a technology for directly connecting a high-output secondary battery and a large-capacity secondary battery in parallel with each other to construct a battery assembly and manufacturing a battery pack with the battery assembly. In this technology, however, different kinds of battery cells constitute one unit module, and therefore, the battery pack cannot be efficiently operated depending upon the operating conditions of a device in which the battery pack is mounted.